In the farm shop
A mold for a bearing that is poured around the shaft.
The following is a reprint of an article by C. T. Schaeffer, which originally appeared in the May, 1930 issue of Farm Mechanics. Article was sent to us courtesy of Dick Hamp, 1772 Conrad Ave., San Jose, CA 95124.
Babbitt metal as used for bearings varies greatly in composition, and its pouring temperature depends upon its composition. Also the grade of babbitt which will be used on repair jobs will depend upon the local stock from which the selection is made. Various babbitt linings contain zinc, lead, aluminum, copper, tin and antimony, but real babbitt is a combination of the last three in various proportions for different grades of service. Where bearings are large and the service is light, the babbitt metal may have a comparatively large percentage of lead and naturally the metal will be comparatively cheap as compared to a high grade metal that will stand high speeds and heavy loads. Lead has many properties which would make it an ideal bearing metal, but it is too soft to stand hard uses in service. However, in the cheaper metals it forms the base when combined with copper, tin, zinc and antimony.
A lead content is generally found in babbitt metal used for general purpose bearings where weight is no object in the design of the unit and parts can be properly proportioned for the service they are to render. Thus, for heavy duty the metal may consist of 40 parts lead, 50 parts tin, 8 parts antimony and 2 parts copper. For light work the specifications may call for 5 parts lead, 44 parts tin, 50 parts antimony and 1 part copper. High grade babbitt used for the bearings of engines generally consists of about 86 percent tin, 7 percent copper and 7 percent antimony. The softer the metal is the less resistant it is and naturally its pouring temperature is lower than that of the harder metals.
Babbitt metal should be placed in a pot or ladle and carefully heated until melted. The metal should also be stirred thoroughly before pouring and care must be taken to melt it slowly. A sufficient quantity of the metal should also be melted to fill the bearing to permit pouring without draining the ladle. This is to insure clean babbitt being used throughout the bearing, leaving the scum, dirt and oxidized metal in the ladle. The molten metal should be kept thoroughly mixed, especially just before pouring, by continually bringing the lower strata up from the bottom of the ladle. If permitted to remain quiet while in the liquid state, the mass may tend to separate into layers. Any lead present, being heaviest sinks to the bottom and the lighter ingredients come to the top. Bearings poured from metal in this state are bound to give trouble.
An old practice is to mix old babbitt from worn bearings with new metal but such practice is not recommended, although in emergencies it may be advisable to use all old metal in preference to waiting for the new metal. Water should not be permitted to drop into the molten metal as this will cause the metal to fly in all directions with danger of burns to the workman. Dross should be removed from the the surface before pouring the metal and upon completion of babbitting operations, the metal, before cooling, should be covered with charcoal or sawdust to retard oxidation and formation of scum.
Various methods have been recommended for determining the correct temperature of the metal for pouring, however, the only safe method is to bring it up to the pouring temperature specified by the maker with allowance sufficient to compensate for the heat loss in transferring the metal to the mould. With lead base babbitt, a fairly accurate method is to heat the metal until it reaches a point where it will just char a soft pine stick. The metal is then ready to pour. The appearance of the metal when skimmed is also used as a means of determining the pouring temperature. When the metal appears like quicksilver and tarnishes slowly after the scum has been skimmed off it is at the correct temperature. Metal that is too hot coats rapidly and the tarnish shows all colors, while metal that is too cold is sluggish and the tarnish takes on a dull appearance. Babbitt metal for bearings must not be overheated.
If it sets the pine stick on fire, or any of the other indications of too hot a metal are evident, the metal has been spoiled for bearing purposes. The better grades of metal which do not contain lead require a higher temperature and it is always necessary to bring the tin and antimony to complete liquefaction. In the case of the metal containing 86 percent tin and 7 percent antimony this starts at about 775 degrees Fahr., and it is necessary to raise this sufficiently to allow for the loss of heat in transferring the metal from the ladle to the mould. This will require an additional 150 to 200 degrees Fahr.
The first step in rebabbitting a bearing is to remove the old metal and then thoroughly clean the part in which the babbitt is to be cast. The old lining of an iron or steel shell can be chipped out or melted out either by torch or forge or by immersion in a pot of old metal. The shell is free from oil or dirt when the smoke ceases, but all traces of rust, dirt and old lining materials must still be removed. Sand, scale and rust should be removed with wire brush or by a pickling bath. Anchor holes or grooves should be well cleaned so that new metal may flow in to form a secure anchor. The old lining from bronze shells can be removed by heating, preferably in a pot of scrap babbitt in which the temperature does not exceed 825 degrees Fahr. The bearing shells after cleaning should have their surfaces scraped with a coarse file as a final precaution and in the case of a tinned bearing, to give a better surface for the bonding of the tin.
Anchoring the babbitt by tinning bonds the metal to its backing; however, the tinning mixture which acts as the fusing medium will adhere only to a surface which has been specially treated. The tin must be made to adhere to the entire contact surface in a thin uniform clear film, otherwise the babbitt liner is apt to crack or loosen. A bright surface is necessary for bonding the tin and this is best obtained by roughing with an old file. Surfaces which do not require tinning should be covered with a thin mixture of graphite and water or red clay and water. The latter should contain only enough clay to produce a red color and should be kept well stirred. As soon as the shell is dry, flux the seat with cut acid. The tinning alloy should be '50-50' solder (half tin and half lead) of high purity. This should be applied while the shell is hot.
In the farm shop it will be more convenient to do the tinning with the tinning alloy in bar form by rubbing the bar over the surface to be tinned. The bar should melt and flow freely and a hot soldering iron can be used to assist in covering all the surface with a good coating. Rubbing with a dry rag, or soft brush will then make the surface even. The tinning process should also include the end surfaces which are to receive the babbitt.
The shaft which is used as the core of the mould may be made of wood although metal is preferable and necessary for very large bearings. To keep the moulten metal from sticking to the shaft it may be wrapped with two or three layers of thin paper or the surface may be coated with a light coat of oil, chalk, or smoked before placing it in position. This precaution is necessary to insure a smooth finish. The clay mixture previously mentioned may also be used. Paper may also be used for gaskets to keep the hot metal from coming in contact with wood which may be used to form the ends of the mould.
The illustrations show several forms of mould recommended by a prominent maker of babbitt metal for casting bearings which are supported in shells and split to permit adjustment. A mould for a bearing that is poured around the shaft is shown in Fig. 1. The end pieces which are of wood are fit to the shaft and held together by rods and nuts. The main requisite in this case is to have the bearing shell spaced evenly around the shaft. This bearing may also be poured in vertical position with the set up shown in Fig. 2. However, in this case a special form of core is necessary or the shaft must be cut in half and fastened to the spacing bars.
To avoid this the set up shown in Fig. 3 may be employed, in which case both halves of the bearing are cast at the same time, that is, in one set up of the mould. The shaft which is to form the core should be mounted in a substantial block of wood in such manner that it will be held perfectly perpendicular and the board covered with paper to serve as a gasket. The two bearing shells are then centered about the shaft with steel liners between them and of a thickness equal to the thickness of the shims which are to be used between the two halves of the bearing when it is finally assembled, plus whatever amount may be allowed for final fitting of the bearing. This applies when the core (shaft in mould) is of the same diameter as the shaft which the finished bearing is to fit. It is best to use shims of equal thickness and make the allowance for fitting in the diameter of the shaft so that the cast bearing will be slightly smaller in diameter than the shaft it is to support and thus allow for complete fitting around all points on the circumference of the bearing surface. Thus the shaft of the mould which serves as the core would be of the same size as the bearing is to support, minus whatever amount may be necessary for final fitting of the bearing.
Care should be taken to position the liners or shims of the mould so they are in contact with the shaft as shown, and securely held in position by drawing up on the 'C' clamp. It will also be necessary to clamp the shells to the base block in some manner so they will be kept in proper relation, that is, centered around the shaft while pouring the bearing. If this is not done there is danger of the shell moving and thus making the bearing liner heavier in one shell than it is in the other. This would also result in misalignment of the bearing when it is finally assembled.
Reference has been made to shrinkage of the metal and this must be avoided before the mould is properly filled. The babitt and its shell should establish full contact and shrink together because intimate contact between the two must exist, otherwise, the turning force of the shaft may tear the babbitt loose from its shell. For this reason preheating of the parts of the mould is recommended. Both bearing shell and shaft should be sufficiently heated at pouring time to permit smooth flow and gradual cooling of the babbitt metal. The mould must not be so hot as to cause the heavier ingredients of the metal to settle to the bottom of the mould. This would result in a bearing having hard and soft spots.
A trace of moisture will cause trouble and the melted metal to be thrown out as soon as it strikes the moisture, with the attendant danger of injury to the workman. Preheating burns off moisture and grease which if allowed to remain would form gases and produce blow holes. All parts of' the mould must be thoroughly dry before the mould is poured, yet, there is no hard and fast rule which can be applied in determining this feature. Iron or steel shells may be considered at proper preheating temperature when a stick of '50-50' solder touched to the surface will just melt. The shaft may be considered at proper temperature when water evaporates rapidly from the surface without sputtering. Preheating of the mould can be done with a blow torch.
The use of a hollow core is sometimes recommended because of the ease with which the temperature can be controlled. Fig. 4 depicts a set up employing this feature, heating of the mould being accomplished with a red hot bar of iron or steel which is inserted in the hollow shaft while the babbitt metal is being melted. Paper gaskets are placed at each end of the mould and the large collars serve to center the shaft and also close the ends of the mould. Vents are provided in each collar and the mould is poured through the oil hole at the top of the bracket. Holes are shown to anchor the babbitt metal and these will have to be covered with paper. There are many similar set ups which will suggest themselves when the part to be babbitted is examined. In this case the bracket can be bolted to the work bench and the collars which complete the mould are rigidly held in position on the hollow shaft with set screws. Regardless of what type of set up may be used, bear in mind that the essential feature is to have the cavity of the mould perfectly clean and dry.
Water should not be permitted to drop into the heated pot of metal as it causes the molten metal to fly in all directions with danger of burns to the operator. Correct temperature of molten metal must next be assured. Unheated metal will give a coarse granular structure, and if overheated, the metal partially oxidizes, becomes dirty, shrinks excessively, softens and has poor anti-frictional properties. While the temperature may be judged by various methods already mentioned, the 'feel' in stirring is also used, although this requires considerable experience. If the composition of the metal does not permit the use of the pine stick, then color must be depended upon, lacking the necessary equipment for exact measurement of temperature. In general it is safe to check the heat when the color changes from a silvery to a yellowish tinge.
Bear in mind that babbitt metal is weak while hot, so that the poured bearing must be handled carefully while cooling and the mould should not be removed until all danger of ' disturbing the metal is over.